The present invention relates to a tape measuring method, and more particularly to a method for measuring a remaining time or an elapsed time for a running magnetic recording tape with a relatively simple circuit arrangement.
It is known in the art of ordinary magnetic tape recording and playback apparatus that there is a certain correlation between the ratio of the rotational periods of tape supply and takeup reels and the remaining time or the elapsed time for a running tape. Thus the following equations are given: ##EQU1## where r is the radius of a hub, r.sub.1 is the radius of a tape roll wound on a supply reel, r.sub.2 is the radius of a tape roll wound on a takeup reel, L.sub.1 and L.sub.2 are the tape lengths on the supply and takeup reels, t.sub.1 and t.sub.2 are the periods of rotation of the supply and takeup reels, and .DELTA.r is the thickness of the tape. The length of the remaining tape can be derived from the above equations (1) through (3) as follows: ##EQU2## where L.sub.0 =L.sub.1 +L.sub.2 (the total length of the tape). The remaining time T for the tape is given as follows: EQU T=L.sub.1 /v (5)
where v is the transfer speed of the tape. The remaining time can be expressed as the curve l.sub.1 (FIG. 1) from the ratio (t.sub.1 /t.sub.2) between the rotational periods of the supply and takeup reels.
The length L.sub.2 of the tape that has transferred can be derived as follows: ##EQU3## The elapsed time for the running tape can be expressed as the curve l.sub.2 (FIG. 1).
One way to measure the remaining time or the elapsed time for the tape from the equation (4) or (5) through measuring the periods t.sub.1, t.sub.2 would be to effect an arithmetic operation using the equation with the periods t.sub.1, t.sub.2 measured. This process would require a computer, could not measure the periods t.sub.1, t.sub.2 simultaneously, and would take an increased interval of time because of the arithmetic operation to be carried out. An interface for allowing input signals to be supplied to a display unit would be complex and costly. Other arithmetic and control circuits would be necessary for simultaneous implementation of a tape counter, a timer switch, and the like.
Another process would be to prepare in advance a data table of numerical data from the equation (4) or (6), and read out of the data table such data which correspond to a remaining time or an elapsed time based on the result of an experiment. This procedure, however, would lead to an extremely large size of such a data table, and hence would be expensive.